(Corrects headline and first paragraph to show plan is for three years, not two)

* New battle plan to set air quality targets for 2020

* Previous plan expired at end of 2017

* To keep tackling scattered pollution sources this year

BEIJING, Jan 31 (Reuters) - China is drawing up plans for tougher curbs on smog during the three years to 2020, an environment ministry official said on Wednesday, after a five-year crackdown on pollution helped it attain air quality targets last month.

"A three-year battle plan in the war to protect blue skies" is being hammered out by officials of the Ministry of Environmental Protection, said spokesman Liu Youbin, and incorporates tougher curbs on major industrial regions.

Key areas facing such restrictions are the Beijing-Tianjin-Hebei zone in the country's north and the Yangtze and Pearl River deltas further south.

The new three-year plan is expected to be completed in the first half of the year, Liu told reporters.

"It will continue to make Beijing-Tianjin-Hebei its key focus, but it will also focus on other major regions like the Yangtze river delta, the northeast and Chengdu-Chongqing," he added.

China's previous action plan for 2013 to 2017 spurred action by the smog-prone Beijing-Tianjin-Hebei region to cut concentrations of hazardous particles known as PM2.5 by more than 25 percent.

The Yangtze and Pearl river delta regions, which include Shanghai and Guangzhou respectively, were pushed to make cuts of 20 percent and 15 percent.

Despite near-record PM2.5 readings in January and February last year, China managed to reach all its 2013-2017 air quality targets by the end of 2017, following efforts to curb coal use and cut output from polluting factories in 28 northern cities in winter.

With the campaign set to end in March, China wants to subject firms to more permanent scrutiny, so as to avert any possible backsliding after 2017 targets were met.

Beijing has already sought to impose new special emissions curbs on major industrial sectors in the north.

Liu said China would continue to battle scattered coal-burning sources, a major source of uncontrolled pollution in provinces like Hebei, and would promote clean energy heating.

This week media reports said Hebei was forced to suspend plans to convert large numbers of coal-fired heating boilers to natural gas after winter supply shortages left thousands of homes without heat.

However, Liu denied the reports, telling reporters the Hebei government would continue to pay close attention to its coal-to-gas conversion efforts.

Beijing could adjust priorities this year and focus on vehicle pollution, Yu Jianhua, chief engineer of the capital's environmental protection bureau, told a separate briefing on Wednesday.

Beijing will keep up the toughest emissions curbs on industrial firms this year, not only to cut pollution but also spur industries to relocate, Yu added.

Car emissions contribute about 40 percent of air pollution in Beijing, the government estimates.

Liu Zhiquan, head of the ministry's monitoring office, said China's average PM2.5 readings from Jan. 1-28 were 64 micrograms per cubic metre, down a fifth on the year.

Concentrations in Beijing fell 70.5 percent to 36 micrograms per cubic metre, close to the state standard of 35 micrograms.

But average readings in the Yangtze and Pearl river deltas rose over the month, he said, without giving details. (Reporting by Muyu Xu and David Stanway; Editing by Joseph Radford and Clarence Fernandez)